Four exhausted Tongan fishermen fought off man-eating fish - thought to be small sharks - and hypothermia as they clung to a makeshift raft for nearly eight hours in rough seas.

They were finally rescued in the dark after flashing a torch at a rescue vessel, but a fifth crew member was not so lucky.

After a bite to his upper arm, the 44-year-old man died within a couple of hours of their fishing boat sinking on Tuesday about 50 kilometres northwest of Nuku'alofa, the capital of Tonga.

His friends tied his body onto a raft they had roped together out of plastic 20-gallon diesel containers and buoys.

It was still hours before a Tongan Navy patrol boat would find the men in the dark, downwind of co-ordinates provided by the National Rescue Coordination Centre in Wellington after a tip-off from an Air New Zealand plane flying over Tonga.

The pilots had reported a distress signal to the Tonga Tower at Fua'amotu Airport, which passed the information to Wellington, where the emergency beacon co-ordinates were soon confirmed by satellite.

The 10m wooden fishing boat Tu'amelie ki Moana had set out from Tongatapu island on Monday in search of big catches above the sea mountains off its western tip.

But after a good haul on Tuesday disaster struck when a rogue wave picked up the vessel and dumped it into the sea.

Lieutenant Siau Fifita, of the Tongan Navy, told the Herald yesterday that the men had immediately realised they were in trouble.

"They had been heading towards the wind at high speed. The crash stopped the engine and the first thing they thought was the bilge [pump], but the bottom of the boat was already filled. They said it sank very, very quickly."

Mr Fifita said the crew managed to set off the Epirb (Emergency position indicating radiobeacon) and grabbed anything that was by then floating, including the torch that would save them.

They hastily made a raft and clung on in rough seas.

Most people spoken to yesterday said the fish had been young sharks less than a metre long. But Mafi Taukitoku, who is in charge of the fishing boat, told a local journalist that the fish were not sharks, but small fish unknown to them.

Mr Fifita said one of the crew had been bitten under the arm by a small shark and later died, possibly from shock. Another man had been repeatedly bitten on his feet, legs, back and stomach by sharks, less than a metre long.

"They would just take mouthfuls, they were not big enough to bite off limbs."

Mr Fifita said he was notified of a possible stranded vessel at about 4pm (New Zealand time) on Tuesday with the go-ahead from Tongan police to leave Nuku'alofa on the Navy patrol boat Voea Savea at 6pm.

The patrol started searching from estimated drift position and was "very lucky" to find the survivors on the trajectory path, he said.

Mr Fifita said it was especially incredible given the difficult conditions - four to six-metre waves and poor visibility in the dark and drizzle - and that the fishing crew were low in the water.

"We were looking for a boat, expecting to tow it in."

Mr Fifita said the crew were only spotted by the flashing of their torch.

"If they had not had that torch we would not have seen them," he said.

"They whistled to us. That was the only energy left in them."

It still took over an hour to get the men on board the 31m patrol boat.

"It was very dangerous to lower the inflatable boat ... very, very risky to pick them up."

Mr Fifita said the injured man was "nearly dead" and another two were extremely weak and would not have survived much longer.